1. Field of the Invention
This invention relates to a pneumatic radial tire having a belt incorporated in the annular tread.
2. Description of Related Art
FIG. 6 shows a conventional pneumatic radial tire which comprises an annular tread b having a belt incorporated therein, a pair of sidewalls c that extend radially inward from the both ends of the annular tread b, and bead portions d which are provided at the inner ends of the sidewalls and each of which has a bead core.
During high-speed running, a strong centrifugal force is exerted on the annular tread b of the tire a, and under this centrifugal force, the annular tread b deforms radially outward as indicated by dashed line b.sub.1, thereby causing "lifting" (an increased outside diameter of the tire).
The amount of tire deformation due to lifting is not uniform in the axial direction (i.e., the width direction of tire) of the annular tread b and, on account of the difference in the rigidity of the belt, the deformation of the shoulder at either end of the tread b is greater than the deformation of the center portion (l.sub.0 &lt;l.sub.1).
The deformation due to lifting also increases the temperature of the shoulder and can potentially destroy the adhesion between rubber and the steel cords in the belt at the shoulder. In addition, the resulting change in the shape of the tire portion that contacts the road surface often causes poor maneuverability at high speed and insufficient grip property.
The conventional means of solving this problem is to improve the belt rigidity by either increasing the diameter or number of belt cords or providing a reinforcement layer which extends radially outward of the belt.
However, these methods increase the tire weight and are unable to meet the requirement for lighter tires, and the demand for a lighter tire is increasing today as the performance of vehicles is constantly improving. Further, an improvement in the belt rigidity leads to a higher tread rigidity and the resulting increase in the longitudinal rigidity of the tire taken as a whole will unavoidably lead to poor ride characteristics.
The increased tread rigidity often causes other problems including higher road noise, lower endurance and poor performance on wet surfaces.